tunnel.go

package main

import (
	"io"
	"net"
	"net/http"
	"time"
)

func handleTunneling(w http.ResponseWriter, r *http.Request) {
	dst, err := net.DialTimeout("tcp", r.Host, 10*time.Second)
	if err != nil {
		http.Error(w, err.Error(), http.StatusServiceUnavailable)
		return
	}

	w.WriteHeader(http.StatusOK)
	hijacker, ok := w.(http.Hijacker)
	if !ok {
		http.Error(w, "Hijacking not supported", http.StatusInternalServerError)
		return
	}

	conn, _, err := hijacker.Hijack()
	if err != nil {
		http.Error(w, err.Error(), http.StatusServiceUnavailable)
	}

	go transfer(dst, conn)
	go transfer(conn, dst)
}

func transfer(dst io.WriteCloser, src io.ReadCloser) {
	defer dst.Close()
	defer src.Close()
	_, _ = io.Copy(dst, src)
}